Turkey, a key Nato ally has established closer co-operation with Russia recently.

The two sides are also at loggerheads over Fethullah Gulen. Turkey accuses the Pennsylvania-based cleric of orchestrating the failed coup and wants him extradited.

Officially Washington insists any decision on returning him to Turkey from the US remains a judicial rather than a political one.

German Chancellor Angela Merkel warned that the “tight referendum result shows how deeply divided Turkish society is and that means a big responsibility for the Turkish leadership and for President Erdogan personally”.

Others expressed concern about the possibility of the return of capital punishment.

The French president’s office warned that any referendum on reviving the death penalty would “obviously be a break with values and engagements” that Turkey had accepted in joining the Council of Europe. The president of the European Parliament, Antonio Tajani, tweeted his own concerns.

Austrian Foreign Minister Sebastian Kurz went further. He said the referendum result was a “clear signal against the European Union. The fiction of Turkey’s bid to join the bloc must be ended,” Mr Kurz said.

Despite saying that the voting day was ‘well administered’, the Organization for Security and Co-operation in Europe (OSCE) and the Council of Europe criticised the referendum campaign, and the Council of Europe said the vote ‘did not live up’ its standards.

The monitors also criticised a late change by electoral officials that allowed voting papers without official stamps to be counted. But the head of Turkey’s electoral body, Sadi Guven, said the unstamped ballot papers had been produced by the High Electoral Board and were valid. He said a similar procedure had been used in past elections.

Mr Erdogan told supporters that Turkey did not “see, hear or acknowledge the politically motivated reports” of the monitors.

He said the result ended the debate on changing the constitution and creating an executive presidency, that the process of implementing the reforms would now begin.

He also said the country could hold a referendum on its long-stalled EU membership bid.

Additionally, Mr Erdogan said he would approve the death penalty if it was supported in a referendum or a bill was submitted to him through parliament. This would end Turkey’s EU negotiations.